1. Field of the Invention
This invention relates to a nonionic modified polyisocyanate (or a polyisocyanate mixture) which has isocyanate groups and has been modified to be hydrophilic. More particularly, it relates to a modified polyisocyanate which can be used advantageously as a curing agent for aqueous resin compositions (e.g., paints, printing inks and adhesives).
2. Description of the Related Art
Modified polyisocyanates have been proposed for the purpose of producing an aqueous two-component polyurethane resin by emulsifying a water-soluble or water-dispersible polyisocyanate into an aqueous polymer solution or aqueous polymer dispersion. For example, JP-A-6-17004 discloses an aqueous coating composition which comprises a mixture of a polyol component comprising an olefin polymer having a hydroxyl group, etc. (a binder) with a polyisocyanate component in which the polyether chain has 5 to 70 in average of ethylene oxide units (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). JP-A-7-109327 discloses a water-dispersible or water-soluble polyisocyanate composition obtained by allowing a monohydric or polyhydric alcohol to react with tolylene diisocyanate, in which at least 70 mol % of 7 to 25 in average alkylene oxide units are ethylene oxide units.
JP-B-4-15270 discloses an adhesive composition which comprises a water dispersion of a water-soluble polymer and an aqueous emulsion and a terminal isocyanate group-containing auto-emulsifying prepolymer which is obtained by allowing an isocyanurate ring-containing hexamethylene diisocyanate polymer to react with an alkoxy polyalkylene glycol having a molecular weight of from 250 to 4,000 (the term "JP-B" as used herein means an "examined Japanese patent application").
JP-B-7-30160 and JP-A-9-137146 disclose a water-dispersible polyisocyanate preparation having an average NCO functionality of from 2 to 3.5, as an additive agent for use in aqueous adhesives which use a polymer dispersed in water as the base. In these references, a reaction product of an aliphatic polyisocyanate with a nonionic polyalkylene ether alcohol which contains at least one polyether chain having at least 10 ethylene oxide units is disclosed as the above-described polyisocyanate preparation, and phenol as well as methanol are described as the alcohol for the production of the polyether alcohol. In this connection, HLB of the nonionic polyalkylene ether alcohol, when phenol is used as a starter, is about 18 or more.
JP-A-5-222150 describes that a polyisocyanate mixture is obtained by allowing a polyisocyanate component to react with a polyalkylene oxide polyether having 5 to 99 in average of ethylene oxide units and that a monohydric alcohol having a molecular weight of from 32 to 150 (particularly methanol) can be used as a starter for obtaining the polyether alcohol.
These polyisocyanates (auto-emulsifying polyisocyanates) can improve emulsifying ability in water. However, since they have small emulsifying and dispersing abilities in aqueous polymer solutions and aqueous polymer dispersions, emulsification of the polyisocyanates (auto-emulsifying polyisocyanates) requires a high shearing force by a mixer, etc. or requires a prolonged period of mixing time when the shearing force is low. Because of their small emulsifying and dispersing abilities, these polyisocyanates do not emulsify or disperse uniformly when added to an aqueous resin composition and they are partially crystallized or precipitated, so that isocyanate cannot be used effectively as a curing agent or cross-linking agent. In addition, when they are used as a curing agent for aqueous paints, transparency and uniformity of the resulting coats are also reduced due to decreased emulsifying and dispersing abilities.
In view of the above, an object of the present invention is to provide a modified organic polyisocyanate which shows high emulsifying and dispersing abilities in aqueous polymer solutions or aqueous polymer dispersions even if it has low emulsifying ability in water itself and can be emulsified therein within a short period of time even under a low shearing force, as well as to a process for producing the same.
Another object of the present invention is to provide a modified polyisocyanate which can be used effectively as a curing agent and a cross-linking agent for aqueous resin compositions (e.g., paints, printing inks and adhesives), and a process for producing the same.